Primary grain size in material produced by a casting process depends on the nucleation frequency and on the growth rate of the first crystals formed during the solidification process. To control the grain size in order to obtain coarse grains, certain elements or compounds are avoided, while other such additives are made in order to obtain a fine grain size. However, when it concerns grain refinement, a quick and reliable method to measure and to control the properties as cast of a certain melt before casting has so far been missing. As a result the additives are often added in amounts that are much larger than what is necessary. Apart from the drawback of the unnecessary high costs of additives, these large additions often lead to problems with large agglomerated particles when recycling the material. Hence, there is a need for a method for obtaining castings comprising small nucleating particles which uses a minimum of grain-modifying additives.
As mentioned above, the addition of nucleating particles to stimulate the formation of crystals upon solidification is well-known. Examples of suitable nucleating particles are boride or carbide particles (aluminium), zirconium (magnesium) and TiC-particles (steel) etc. In many cases, it is also possible to control the growth parameter of crystals in solidifying metal melts.
As already mentioned, the present invention relates to optimising the grain refinement of aluminium alloys. It is based upon controlled additions of agents promoting grain refinement of aluminium, such as the elements Ti, Zr, B, N and C, mostly in the form of master alloys, which are added to the molten metal.
The master alloys are usually added in the form of small buttons or ingots, or when continuous additions are desirable (as in direct chill casting of billets or slabs) the addition is made by feeding a rod into the flowing melt stream. Various master alloy compositions and methods of manufacture and use have been proposed. (See, for example, patents U.S. Pat. Nos. 3,785,807, 3,933,476, 4,298,408, 4,612,073, 4,748,001, 4,812,290 and 5,055,256).
It should be stressed that all aluminium-titanium-boron (Al--Ti--B) master alloys, regardless of their composition, are a mixtures of two crystals interspersed in a matrix of solidified aluminium. These two phases are titanium diboride (Al,TiB.sub.2) and titanium aluminide (TiAl.sub.3). The whole range of boride particles from AlB.sub.2 -TiB.sub.2 may form during production of master alloys.
In alloys with excess Ti compared to what is needed to form TiB.sub.2 most boride particles have a composition close to TiB.sub.2. For the sake of simplicity this phase is considered in the following text.
Virtually all of the titanium and boron in master alloy grain refiners are contained in these crystals, because the solubility of boron and titanium in solid aluminium at room temperature is very small. This means that changing the master alloy composition only changes the relative proportion of these two crystals which are added to affect the grain refinement.
In spite of this simple fact, there has been an enormous amount of controversy, and disagreement about what Ti to B ratio the master alloy should contain for best grain refinement. This question was considered at some length in U.S. Pat. No. 4,612,073 and also in the paper by M. M. Guzowski, G. K. Sigworth and D. A. Sentner entitled "The Role of Boron in the Grain Refinement of Aluminium" (published in Metallurgical Transactions, vol. 18A, 1987, on pages 603-610). The view taken by Guzowski et al. was that boron acts to change the shape of the TiAl.sub.3 crystal and that TiB.sub.2 can also be an effective nucleant when there is a significant amount of dissolved Ti in the melt. This question (of the optimum Ti/B ratio) has also been addressed in an empirical fashion, by doing extensive grain refining tests, and then using the measured results to "map out" desired grain refining practises. A typical example of this approach is the paper entitled "Grain Refining Response Surfaces in Aluminium Alloys", which as published by W. C. Setzer et al. on pages 745-748 of Light Metals (1989).
In spite of the importance of this question, there is no understanding of what the optimum titanium to boron ratio should be, for any particular alloy and for a specific casting process. Over the years, our empirical knowledge has led us to realise that the best grain refiner for one alloy may not be the best for another alloy. Commercial alloys range from relatively pure aluminium (such as foil and electrically conducting wire) to casting alloys which may contain nearly 20% of dissolved elements. It has been found that master alloys which grain refine well in pure aluminium do not usually work in highly alloyed melts, and vice versa. (See U.S. Pat. No. 5,055,256, where a master alloy composition has been disclosed solely for aluminium-base alloys containing high Si contents).
Several methods based on thermal analysis have been proposed to monitor the grain refining process (U.S. Pat. No. 3,785,807; Apelian et al., AFS Transactions, 84-161, p. 297-307). However, none of these methods can be generally applied, as one single temperature/time curve cannot separate the two phenomena of nucleation and growth as independent processes.
This situation means that in many cases the grain refiner practice used in the cast shop is far from the optimum procedure. At best, one is perhaps using too much grain refiner, and thereby spending too much for the master alloy. At worst, one can run into casting problems, such as cracking or other defects in the finished cast product.